This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-217531, filed Jul. 18, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an apparatus for optically transmitting data between a rotor and a stator and an X-ray CT apparatus having the apparatus incorporated therein.
2. Description of the Related Art
Known is an optical transmission apparatus for transmitting data by utilizing an optical beam transmitted in the space between a rotor and a stator. In the optical transmission apparatus, a cable is not used and, thus, the rotation of the rotor is not restricted so that the rotor can be freely rotated. In addition, a mechanical contact or contacts are not required in the optical transmission apparatus unlike the transmission apparatus using a slip ring, leading to a high reliability.
The particular optical transmission apparatus is used, for example, in an X-ray CT (computed-tomography) apparatus, which is one of medical image diagnostic apparatuses. The X-ray CT apparatus comprises a hollow rotor and a hollow stator, and a patient (human body) is put in the bore of the X-ray CT apparatus. An X-ray tube and an X-ray detector are arranged in the rotor, and the patient put in the X-ray CT apparatus is irradiated with an X-ray emitted from the X-ray tube while rotating the rotor to permit the X-ray to scan, for example, spirally the patient. The X-ray passing through the patient is detected by the X-ray detector so as to be taken out as an electric signal. This electric signal is converted into a digital signal and, then, converted again into a light beam so as to be transmitted to the side of the stator. The light beam transmitted to the side of the stator is received by a photo-detecting element and is converted into an electric signal, and the electric signal is processed to obtain a tomographic image information or data of the patient.
In the optical transmission apparatus between the rotor and the stator utilized in the conventional X-ray CT apparatus, a plurality of light-emitting elements 83, e.g., 64 light-emitting elements 82, are arranged at a predetermined interval on a circle having a predetermined radius on a rotor 81, as shown in FIG. 1. For simplifying the drawing, only two light-emitting elements 83a and 83b are shown in FIG. 1. Also, a single photo-detecting element 84 is arranged on a circle having the radius equal to that noted above on a stator 82. A light beam modulated in accordance with the image data is transmitted between the light-emitting element 83 and the photo-detecting element 84. In accordance with rotation of the rotor 81, the light-emitting elements 83a and 83b are successively faced to the photo-detecting element 84. FIG. 1 shows the state immediately after the light-emitting element 83a has being faced to the photo-detecting element 84 and the state immediately before the light-emitting element 83b reaches the photo-detecting element 84.
Light beams 85a, 85b emitted from the light-emitting elements 83a, 83b is diverged, respectively. In the state shown in FIG. 1, the photo-detecting element 84 is positioned intermediate between the light-emitting elements 83a and 83b so as to receive the edge portions of the light beams 85a, 85b, i.e., the weak portions of the light power. However, since the photo-detecting element 84 simultaneously receives the two light beams 85a, 85b, the power of the received light beams (incident light intensity) is maintained at a relatively high level.
In order to ensure a sufficiently high optical power even in the case where the photo-detecting element 84 is positioned intermediate between the light-emitting elements 83a and 83b, it is necessary to set the distance between the light-emitting elements 83a and 83b such that the light beams 85a, 85b emitted the light-emitting elements 83a, 83b are allowed to partially overlap each other, as shown in FIG. 1. Because of this requirement, the number of light-emitting elements is increased. For example, it is necessary to arrange 64 light-emitting elements. In this connection, the number of IC""s for driving the light-emitting elements and the wiring for connecting these IC""s is increased, leading to serious problems. For example, the power consumption is increased. In addition, the reliability is lowered and the manufacturing cost is increased.
In order to avoid the problems pointed out above, it may be advisable to increase the distance between the rotor 81 and the stator 82 so as to widen the distribution of the light beams emitted from the light-emitting elements and, thus, to decrease the number of photo-detecting elements. It should be noted in this connection, however, that the intensity of light is inversely proportional to the square of the distance. In other words, if the distance between the rotor 81 and the stator 82 is increased, the power of light received by the photo-detecting element is weakened so as to give rise to a new problem that a transmission error is increased.
The X-ray CT apparatus is required in recent years to transmit a large amount of data at a high speed by the shortening of the irradiating time and the employment of a multi-slice system for acquiring a large number of tomographic images simultaneously. In this connection, transmission of a large amount of data at a high speed is required in respect of the optical transmission apparatus between the rotor and the stator. However, it is difficult for the conventional construction described above to satisfy the particular requirement because of the reasons described below.
Concerning the specific parts used in the optical transmission apparatus between the rotor and the stator, it is possible to use a light-emitting diode (LED) or a laser diode (LD) as the light-emitting element. In the case of transmitting the data of a large capacity roughly exceeding hundred Mbps at a high transmitting speed, an LD is used in many cases. On the other hand, a photodiode (PD) is generally used as the photo-detecting element. The LD and the PD have characteristic parameters called the maximum light output and the minimum light receiving sensitivity, respectively. An allowable loss called optical power budget is determined by the values of these characteristics parameters.
In order to achieve an error-free data transmission by the light beam between the rotor and the stator, it is necessary to make the optical power budget as large as possible. On the other hand, in view of the increase in the data transmission, the light output of the LD tends to be decreased with increase in the data transmission speed. Also, in order to increase the data transmission speed for the PD, it is necessary to decrease the parasitic capacitance so as to diminish the light receiving area, with the result that the received optical power is decreased. It follows that the optical power budget is diminished with increase in the data transmission speed.
It should be noted that, if the transmission speed is increased with the distance between the adjacent light-emitting elements set constant, the light receiving power of the photo-detecting element is diminished in accordance with the transmission speed. Therefore, it is necessary to arrange the light-emitting elements at a higher density in order to ensure a desired light receiving power so as to make the above-noted problems more serious in respect of the increase in the power consumption accompanying the increase in the number of light-emitting elements used as well as the reduction in the reliability and the increase in the manufacturing cost.
As described above, in the conventional light transmission apparatus between a rotor and a stator for performing the data transmission directly by the light beam from a plurality of light-emitting elements arranged in the circumferential direction on the side of the stator toward the photo-detecting element arranged on the side of the stator, it is necessary to arrange a large number of light-emitting elements on the side of the rotor, leading to the problems such as the increase in the power consumption, the reduction of the reliability and the increase in the manufacturing cost. Also, where it is intended to achieve the transmission of a large capacity of data at a high speed as required for an X-ray CT apparatus, the light receiving area of the photo-detecting element is diminished so as to decrease the optical power budget. Therefore, it is necessary to arrange the light-emitting elements at a higher density so as to make the above-noted problems more serious.
An object of the present invention is to provide a light transmitting apparatus between a rotor and a stator capable of effectively decreasing the number of light-emitting elements so as to lower the power consumption, to improve the reliability and to achieve the cost reduction, thereby making the apparatus adapted for the large capacity data transmission at a high speed.
Another object of the present invention is to provide an X-ray CT apparatus provided with the particular light transmitting apparatus between the rotor and the stator.
According to one aspect of the present invention, there is provided an apparatus for transmitting a light beam modulated in accordance with data to be transmitted between a rotor and a stator, the rotor and stator facing each other, and coaxial first and second reference circles having first and second different radii being defined on the rotor and stator respectively, the apparatus comprising:
a light-emitting element configured to emit a light beam in accordance with the data to be transmitted, the light-emitting element being mounted on the rotor and arranged on the first reference circle;
a photo-detecting element configured to detect the light beam emitted from the light-emitting element, the photo-detecting element being mounted on the stator and arranged on the second reference circle; and
an optical guide member configured to guide the light beam emitted from the light-emitting element to the photo-detecting element, the optical guide member being arranged between the stator and the rotor, made of a transparent material, and fixed to one of the stator and the rotor.
According to another aspect of the present invention, there is provided an apparatus for transmitting a light beam modulated in accordance with data to be transmitted between a rotor and a stator, the rotor and stator facing each other, and coaxial first and second reference circles having first and second different radii being defined on the rotor and stator respectively, the apparatus comprising:
groups of light-emitting elements, each group configured to emit a light beam in accordance with the same data to be transmitted, the light-emitting elements being mounted on the rotor and arranged at predetermined intervals on the first reference circle;
photo-detecting elements corresponding to the groups of the light-emitting elements and each configured to detect the light beams emitted from each of the groups of the light-emitting elements, the photo-detecting elements being equidistantly mounted on the stator and arranged on the second reference circle, and the groups of the photo-detecting elements being equal in number to the groups of the light-emitting elements; and
optical guide members each configured to guide the light beam emitted from the group of the light-emitting element to the corresponding photo-detecting element, the optical guide members being arranged between the stator and the rotor, made of a transparent material, and fixed to one of the stator and the rotor.
According to further aspect of the present invention, there is provided an X-ray CT apparatus comprising:
a X-ray tube configured to emit X-rays;
a rotor configured to support the X-ray tube, on which a first reference circle having a first radius is defined;
a stator configured to rotatably support the rotor, on which a second the rotor reference circle having a second radius is defined, the first and second circles being coaxially arranged and the stator facing to the rotor;
a driving mechanism configure to rotate the rotor with the X-ray tube;
a light-emitting element configured to emit a light beam in accordance with the data to be transmitted, the light-emitting element being mounted on the rotor and arranged on the first reference circle;
a photo-detecting element configured to detect the light beam emitted from the light-emitting element, the photo-detecting element being mounted on the stator and arranged on the second reference circle; and
an optical guide member configured to guide the light beam emitted from the light-emitting element to the photo-detecting element, the optical guide member being arranged between the stator and the rotor, made of a transparent material, and fixed to one of the stator and the rotor.
To be more specific, the optical guide member is formed of, for example, a fan-shaped transparent plate having an arcuate portion and a proximal end portion, and includes a tapered first reflecting surface formed in the arcuate portion at a predetermined angle relative to the optical axis of the light-emitting element, and a second reflecting surface formed in the proximal end portion substantially in parallel to the first reflecting surface.
According to one embodiment of the present invention, the optical guide member formed of a fan-shaped transparent plate is fixed to the stator to permit the arcuate portion to be located in a position of a first reference radius and to permit the proximal end portion to be located in a position of a second reference radius such that the light beam emitted from the light-emitting element is successively reflected in the first reflecting surface and, then, in the second reflecting surface so as to guide the light beam to the photo-detecting element.
According to another embodiment of the present invention, the optical guide member formed of a fan-shaped transparent plate is fixed to the rotor to permit the arcuate portion to be located in a position of the second reference radius and to permit the proximal end portion to be located in a position of the first reference radius such that the light beam emitted from the light-emitting element is successively reflected in the second reflecting surface and, then, in the first reflecting surface so as to guide the light beam to the photo-detecting element.
As described, in the present invention, a major portion of the light beam emitted from the light-emitting element is transmitted to reach the photo-detecting element through the optical guide member, making it possible to ensure a sufficient received light power in the photo-detecting element without arranging a large number of light-emitting elements at a small interval. Also, since the reduction of the received light power is small even where the light-emitting elements and the photo-detecting element are arranged apart from each other, it is possible to achieve a low power consumption, to improve the reliability and to achieve the cost saving by decreasing the number of light-emitting elements. Further, even if the light receiving area of the photo-detecting element is diminished in an attempt to increase the operating speed, it is possible to ensure a sufficient received light power so as to make it possible to achieve the data transmission by utilizing the light beam low in transmission error.